Medical thermometer

ABSTRACT

An improved medical thermometer is disclosed that provides an accurate estimate of a patient&#39;s temperature in substantially reduced time as compared to prior thermometers of this kind. This improved performance is achieved by configuring the thermometer to include a hollow, thin-walled metallic probe tip sized for secure attachment to the remote end of an elongated base. This defines an elongated cavity within the probe tip, and a thermistor is bonded to the probe tip within that cavity. The cavity is configured to be substantially longer in the direction of the probe&#39;s longitudinal axis than it is in a transverse direction, to inhibit the conduction of heat along the probe tip to the elongated base. The temperature of the thermistor, therefore, closely follows the temperature of any surface that contacts the metallic tip.

This is a division of application Ser. No. 08/859,050, filed May 20,1997, now U.S. Pat. No. 6,000,846 which is a division of applicationSer. No. 08/333,958, filed Nov. 3, 1994 and now U.S. Pat. No. 5,632,555,which is a continuation-in-part of application Ser. No. 08/303,344,filed Sep. 9, 1994 and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to medical thermometers and, moreparticularly, to electronic thermometers that estimate, or predict, apatient's temperature based on a series of samples of a probe-mountedtemperature sensor.

Medical thermometer's of this particular kind have been in common use inthe clinical environment for many years. The thermometers typicallyinclude an elongated probe configured for convenient oral, rectal, oraxillary use, with a thermistor mounted within the probe's remote tip.In use, a hygienic, plastic probe cover is placed over the probe, andthe probe is then applied to its appropriate location on the patient,whereupon the temperature of the probe and thermistor begin to risetoward the patient temperature. The thermometer periodically samples thethermistor signal and, using one of several known algorithms, predictsthe thermistor's eventual temperature. This temperature prediction isdisplayed long before the thermistor's temperature actually reaches thatpredicted temperature.

Various prediction algorithms have been used in the past, all of themproviding reasonably accurate temperature predictions as quickly asabout 30 seconds after the thermometers are applied to the patients.This represents a marked improvement over the time delays encounteredusing more traditional glass thermometers, which typically are on theorder of about 3 minutes. The time delay is primarily due to the heatcapacity of the probe and the fact that applying the probe to thepatient, e.g., beneath the tongue, draws down the temperature of thetissue in the immediate region of the probe.

Although prior prediction-type electronic thermometers have proven to behighly successful in the clinical environment, there is still a need fora further improved thermometer that can provide accurate predictions ofa patient's actual temperature in substantially less time than generallywas achievable in the past. At the same time, however, the thermometermust not sacrifice accuracy for speed and must be of durableconstruction able to withstand frequent use on multiple patients. Thethermometer also must be substantially insensitive to variations in theparticular manner in which the thermometer is applied to the patient.The present invention fulfills these needs.

SUMMARY OF THE INVENTION

The present invention is embodied in an improved prediction-type medicalthermometer configured to accurately estimate a patient's temperature insubstantially less time than was previously achievable. The thermometerincludes an elongated probe having a hollow metallic tip, with atemperature sensor, e.g., a thermistor, bonded to an inside wall of thatprobe tip, for generating an electrical signal that varies according tothe sensor's temperature. The thermometer further includes an electricalheater, preferably separate from the temperature sensor and bonded tothe inside wall of the probe tip at a location spaced circumferentiallyfrom the temperature sensor. An electrical circuit selectively applies acurrent to the electrical heater, which can take the form of a resistor,to warm the probe tip to a selected temperature in advance of its beingapplied to the patient. This substantially reduces the time required bythe thermometer to accurately estimate the patient's temperature.

In a more detailed feature of the invention, the probe further includesan elongated base, and the hollow metallic tip is sized to be attachedsecurely to that base. The tip is generally cylindrical and formed ofstainless steel having a substantially uniform thickness of less than orequal to about 0.1 millimeters. The temperature sensor and theelectrical heater are preferably bonded to the inside wall of the probetip at substantially diametrically opposed locations.

In another feature of the invention, the electrical circuit for applyinga current to the electrical heater within the probe tip includes aprocessor configured to measure the temperature at a start time, priorto receipt of the probe by the patient, and to apply to the electricalheater an initial electrical signal having a prescribed parameter, e.g.,duration, that varies according to that start time temperature. Theelectrical signal parameter also can be made to vary according to thevoltage of the thermometer's electrical power source. The start timetemperature conveniently can be measured by measuring the temperaturesensor signal. After application of the initial electrical signal, theprocessor continues to apply an electrical signal, e.g., pulse-widthmodulated pulses, to the electrical heater, to controllably adjust thetemperature sensor's temperature to a selected value, e.g., 93° F.,until the probe is applied to the patient.

After the probe is applied to the patient, the processor repeatedlysamples the temperature sensor signal, e.g., at regular time intervals,and estimates the patient's temperature based on a plurality ofsuccessive samples. The processor terminates its estimating andconditions a display to display the most recent temperature estimatewhen a prescribed set of conditions has been met, that prescribed set ofconditions varying in accordance with the values of the successivetemperature estimates. For example, if the most recent estimateindicates that the temperature lies within a prescribed normaltemperature range, e.g., 97° F. to 99.5° F., and if a first selectednumber, e.g., four, of successive estimates are within a firstpredetermined temperature error range, e.g., a span of 0.2° F., then theprocessor terminates its processing and conditions the display todisplay the processor's determination of the patient's temperature,which is the most recent temperature estimate. On the other hand, if themost recent estimate of patient temperature lies outside that normaltemperature range, then the processor continues to sample thetemperature sensor signal and to provide repeated temperature estimatesuntil a second selected number of successive estimates, e.g., six, liewithin a second predetermined temperature error range, e.g., a span of0.25° F. Thus, when the patient appears to have a temperature that mightindicate the need for a therapeutic intervention, the thermometerterminates its measurement process and displays its best estimate ofpatient temperature only after additional measurements have been made.Prior to terminating the estimating process, the processor can conditionthe display either to remain blank or to display the successivetemperature estimates.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred embodiment,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a medical thermometer embodying theinvention, including an elongated probe configured for application to apatient.

FIG. 2 is a fragmentary, longitudinal cross-sectional view of theelongated probe of FIG. 1.

FIG. 3 is a cross-sectional view of the probe's hollow tip, takensubstantially in the direction of the arrows 3--3 in FIG. 2.

FIGS. 4(A) and 4(B) together depict a simplified flowchart showing theoperational steps performed by a microprocessor in preliminarily heatingthe probe tip and estimating the patient's temperature based on a seriesof thermistor signal samples.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the drawings, and particularly to FIGS. 1-3, thereis shown a prediction-type electronic thermometer 11 configured toaccurately estimate a patient's temperature. The thermometer includes abase housing 13 and an elongated probe 15 connected together by aflexible wire 16. When not in use, the probe can be stored convenientlyin a well 17 formed in the base housing. In use, a hygienic probe cover(not shown) selected from a probe cover supply 18 is placed over theprobe, and the probe is applied to a patient, e.g., orally or rectally.The probe includes a thermistor 19 within its remote tip, and electricalcircuitry in the base housing 13 monitors the thermistor and estimatesthe patient's temperature in substantially less time than previously wasrequired by thermometers of this kind. The final temperature estimate isdisplayed on a display 20 mounted on the base housing.

The elongated probe 15 includes an elongated base 21 and a hollow probetip 23 configured for secure attachment to the base. The tip is formedof stainless steel having a substantially uniform thickness of about 0.1millimeters, and it includes a cylindrical section 25 that secures tothe base and a frusto-conical section 27 at its remote end. Thethermistor 19 is bonded to the inside wall of the frusto-conical sectionusing a thermally-conductive epoxy 28, such as Stycast #2850.

Stainless steel has relatively poor thermal conductivity; however, itshigh strength allows the probe tip 23 to be made very thin so that heatcan be conducted from the patient to the thermistor 19 relativelyquickly. The thinness of the wall, coupled with the material'srelatively poor heat conductivity, also provides the advantage ofreducing the wicking of heat axially along the probe tip, whereby heatflow to the thermistor is further enhanced.

The open end of the cylindrical section 25 of the probe tip 23 is sizedto slide over and be retained by the remote end of the probe's base 21.The tip and base can advantageously be secured together using Ecco Bond#51 epoxy. The base likewise is formed of stainless steel and istubular, but with a wall thickness of preferably about 0.4 millimeters.Electrical leads 29 connect the thermistor 19 with the electricalcircuitry located in the base housing 13. These leads extend through thetubular openings in the probe tip 23 and probe base 21. To furtherreduce the conduction of heat away from the thermistor, short sectionsof the leads at the site of the thermistor are formed of nickel, whichhas relatively poor heat conductivity. The remaining sections of theleads are formed of copper. The nickel and copper lead sections aresecured to each other by connectors 30.

To reduce the temperature draw down of the thermistor 19 when the probe15 is placed in the patient's mouth, the thermometer 11 further isconfigured to preliminarily warm the probe tip to a temperature of about93° F. prior to its insertion. This is accomplished using a resistor 31bonded to the inside wall of the frusto-conical section 27 of the hollowprobe's tip 23. The resistor is bonded using a thermally conductiveepoxy 33, such as Stycast #2850, at a circumferential locationdiametrically opposed to that of the thermistor. To reduce the powerload, this warming is effected only upon removal of the probe from itsstorage well 17 in the base housing 13. Electrical current is applied tothe resistor via leads 35.

To warm the probe tip 23 as rapidly as possible, a substantiallycontinuous pulse of electrical current is initially applied to theresistor 31, for a controllably selected time duration, typically on theorder of 1 to 2 seconds. The specific time duration is selectedaccording to the amount of warming determined to be required, which ofcourse depends upon the probe tip's initial temperature at the time itis withdrawn from the well 17. The thermometer 11 therefore isconfigured to measure this initial temperature and to determine thedifference between that measured temperature and the desired 93° F.target temperature. The initial temperature preferably is measured usingthe thermistor 19. Alternatively, it could be measured using a separatethermistor mounted within the base housing 13, preferably adjacent tothe probe cover supply 18.

The appropriate duration for the initial warming pulse is selected bynormalizing the desired temperature rise to the probe's knowntemperature rise undergone when a pulse of a prescribed fixed durationis applied, as determined in a prior test conducted when this same probe15 was first attached to the base housing 13. Thus, for example, if itis known that a pulse duration of precisely 200 milliseconds willincrease the probe tip's temperature from 73.0° F. to 77.0° F., a spanof 4.0° F., then it is determined that a pulse duration of about 900milliseconds will be required to increase the probe's temperature to 93°F. from an initial start temperature measured to be 75° F.

The electrical power delivered to the resistor 31 during the initialwarming pulse can vary according to the voltage level of the battery(not shown) located within the base housing 13. If that voltage isrelatively low, for example, then a proportionately longer pulseduration will be required to provide the desired heating. Thethermometer 11 therefore is configured to measure the battery voltagewhile a warming pulse is being applied and to adjust the pulse duration,accordingly, to provide the desired warming.

It will be appreciated that the warming function of the resistor 31could alternatively be provided by the thermistor 19, itself In thatcase, care must be taken to ensure that the thermistor temperature ismeasured only after the transient effects of any warming pulse appliedto it have adequately diminished.

The thermometer 11 preferably includes a fail-safe circuit (not shown)that monitors the electrical signal applied to the resistor 31 andintervenes to terminate the signal if it is detected to be presentcontinuously. The initial pulse signal applied to the resistor isperiodically interrupted for brief durations, e.g., one millisecond,thus making it only substantially continuous, as mentioned above. Thisperiodic interruption ensures that the fail-safe circuit does notmistake the pulse signal for a failure and intervene to terminate thesignal.

After the probe tip 23 has been warmed to approximately the 93° F.target temperature after removal of the probe 15 from the well 17 of thebase housing 13, the thermometer 11 operates in a sustain mode, in whichit endeavors to regulate the probe tip's temperature at the desired 93°F. value. This is achieved by configuring a microprocessor (not shown)that is part of the electrical circuitry to periodically read thethermistor 19, e.g., every 200 milliseconds, to ascertain its currenttemperature, and to provide pulse-width modulated pulses to the resistor31. The pulse duration of each successive pulse is determined by theformula set forth below, which incorporates 1) a temperature differenceor error value, 2) a temperature slope value, and 3) an integratedtemperature error value: ##EQU1## The constants K₁, K₁ ', K₂, K₂ ' andK₃ all are derived empirically, based on the particular probe structurebeing used. The third term in the equation can be limited to apredetermined maximum value. Those skilled in the art are readilycapable of deriving an appropriate equation.

This pulse-width modulation control scheme is effective in maintainingthe probe tip's temperature at the desired 93° F. even when the probe 15receives the thermal shock of having a hygienic probe cover placed overit. When that occurs, the probe tip's temperature can be reducedsubstantially, which causes the temp error and temp slope terms in theabove equation to increase substantially. This results in pulses ofincreased width being applied to the resistor 31, to rapidly bring theprobe temperature back to the desired 93° F. value.

The probe 15 is placed into the patient's mouth typically at least 5seconds after the probe has been removed from the well 17 of the basehousing 13. At that time, the temperature of the probe tip 23 and thesurrounding probe cover should be at or near the 93° F. targettemperature. That is only slightly below the expected mouth temperature,so that very little draw down of the temperature of the patient's mouthtissue will occur. This is important in minimizing the time delay to theaccurate estimation of the patient's temperature.

When the probe tip is placed into the patient's mouth, its temperatureshould almost immediately rise above the 93° F. target value, whichshould cause the thermometer 11 immediately to reduce to zero durationthe pulses it had been applying to the resistor 31. Thereafter, themicroprocessor continues to sample the thermistor 19 every 200milliseconds and, after alternate samples (thus, every 400milliseconds), analyzes the successive temperature samples and endeavorsto curve-fit those samples to the curve for a typical patient. Numerousprediction algorithms are known and are suitable for this purpose,although a least mean square error curve fit is preferred. The curve forthe typical patient is previously derived based on tests performed on alarge number of individuals.

The microprocessor terminates its sampling of the thermistor 19 andestimating of the patient's temperature only when a prescribed number ofsuccessive temperature estimates, which are made every 400 milliseconds,are sufficiently close to each other to provide at least a limitedmeasure of confidence that the estimate is indeed correct. In onefeature of the invention, a larger number of such consecutivetemperature estimates falling within a predetermined temperature span isrequired when that estimate is below a selected temperature, e.g., 97°F., or above higher predetermined temperature, e.g., 99.5° F. In thosecircumstances, a therapeutic intervention could be indicated, so it isimportant to exercise greater care in ensuring that the temperatureestimate is indeed correct.

Thus, in the preferred embodiment, at least 3.6 seconds must haveelapsed since the start of the prediction process and six consecutivetemperature estimates must lie within 0.25° F. of each other before theprocessor will terminate its estimating function and display on thedisplay 20 the most recent estimate, when that estimate indicates atemperature less than 97° F. or greater than 99.5° F. On the other hand,a minimum time duration of 1.2 seconds and only four consecutiveestimates lying within 0.2° F. of each other are required when atemperature between 97° F. and 99.5° F. is indicated. It will beappreciated that the two above sets of conditions for terminating theprocessor's estimating function are exemplary, only. More than twodifferent sets of conditions alternatively could be used.

Preferably, the display 20 is conditioned to provide a display of onlythe final temperature estimate, and it remains blank while thesuccessive estimates are being computed. Alternatively, however, thedisplay could be conditioned to provide a display of all of thesuccessive temperature estimates, and some means of alerting theoperator, e.g., a beeper, could be provided when the estimating functionhas been completed.

FIGS. 4(A) and 4(B) depict a simplified flowchart of the operationalsteps performed by the microprocessor in controllably heating the probetip 23 after its removal from the well 17 of the base housing 13 and,thereafter, in sampling the thermistor signal and estimating thepatient's temperature. In an initial step 101 of the flowchart, theprocessor applies an initial heating pulse to the resistor 31. Asmentioned above, this step entails measuring the thermistor's initialtemperature upon removal of the probe 15 from the well, as well asmeasuring the voltage on the battery located within the base housing.The pulse duration is controllably adjusted according to these twomeasurements. Thereafter, in step 103, the processor applies a second,sustain pulse to the resistor, which has a duration calculated tosustain the thermistor's temperature at about 93.0° F. when it isrepeated every 200 milliseconds.

In a subsequent step 105, a decrementing predict clock is set to 200milliseconds, to initiate the warming pulse cycle. Thereafter, theprogram remains in step 107 until the predict clock has timed out. Thepredict clock then is reset to 200 milliseconds in step 109, and thethermistor signal is measured in step 111. Another sustain pulse of thesame duration as the first sustain pulse is applied to the resistor 31in step 113, and it is then determined in step 114 whether or not thechange in the thermistor's temperature since the previous measurement isless than 0.2° F. Such a condition would indicate that the thermistor'stemperature has generally stabilized at some temperature at or near 93°F. In this initial pass through step 114, only one thermistormeasurement is available, so the condition automatically is not met, andthe program therefore returns to step 107, where it remains until thepredict clock has timed out.

The program then repeats this open-loop sustain pulse cycle byproceeding again through steps 109, 111, 113 and 114, until it finallyis determined in step 114 that the thermistor's temperature hasadequately stabilized. When that occurs, the program proceeds to step115, where it waits for the predict clock to time out, and in turn tostep 117, where it again sets the predict clock to 200 milliseconds.Then, the thermistor signal is again measured in step 119, and it isdetermined in step 121 whether or not 1) the current temperaturemeasurement exceeds 94.5° F. or 2) the current temperature measurementexceeds 91.5° F. and, at the same time, a pulse width of zero durationis computed using the formula set forth above. These conditionsordinarily would be met only after the probe 15 has been inserted intothe patient's mouth.

If neither of the conditions set forth in step 121 has been met, theprogram proceeds to step 139, where it computes the temp error, tempslope, and integrated temp error variables, and calculates theappropriate pulse width using the formula set forth above. Thiscalculated pulse width then is applied to the resistor 31, in step 141,and the program returns to step 115, where it waits for the predictclock to time out. The program then proceeds again through the steps117, 119, and 121.

Eventually, one of the two conditions set forth in step 121 will be met,which ordinarily will occur only after the thermometer probe 15 has beenplaced into the patient's mouth. When this occurs, the program proceedsto step 123, where it determines whether or not an even number ofsettings of the predict clock have been made. This is required becausepatient temperature estimates are produced only after alternate readingsof the thermistor 19. If not, meaning that an odd number of suchsettings have been made, then the program returns to step 115, asdescribed above.

When it is determined at step 123 that an even number of predict clocksettings have been made, the program proceeds to step 125, where itimplements a prescribed prediction algorithm to estimate the patient'stemperature based on the accumulated temperature samples. Thereafter, instep 127, it is determined whether or not the time since starting theprediction process has exceeded 1.2 seconds. If it has not, the programreturns to step 115, where it remains until the predict clock hasdecremented to zero. Thus, seven thermistor samples and four temperatureestimates must be made before 1.2 seconds have elapsed.

If, on the other hand, it is determined at step 127 that the time periodsince starting the prediction process has in fact exceeded 1.2 seconds,then the program proceeds to step 129 where it is determined whether ornot the current temperature estimate lies within a relatively normalrange of 97° F. to 99.5° F. If it does, then the program proceeds tostep 131, where it is determined whether or not the difference betweenthe maximum and minimum temperature estimates during the preceding 1.2seconds (i.e., seven 200 millisecond samples) is less than 0.2° F. If itis, then it is determined that the current temperature estimate isvalid, and the program proceeds to step 133 of displaying thattemperature estimate.

On the other hand, if it is determined at step 129 that the currenttemperature estimate lies outside the 97° F. to 99.50F. range, or if itis determined at step 131 that the difference between the maximum andminimum temperature estimates exceeds 0.2° F., then the program proceedsto step 135, where it is determined whether or not the time sincestarting the prediction process has exceeded 3.6 seconds. If it has not,the program returns to step 115, as described above. Thus, for patienttemperatures outside the relatively normal range of 97° F. to 99.5° F.,the thermistor sampling and temperature estimating will continue for atleast 3.6 seconds.

When it is finally determined in step 135 that the time since startingthe prediction process has exceeded 3.6 seconds, then the programproceeds to step 137, where it is determined whether or not thedifference between the maximum and minimum temperature estimates duringthe preceding 2.0 seconds (i.e., eleven 200-millisecond samples) is lessthan 0.25° F. If it is, then it is determined that the currenttemperature estimate is valid and the program proceeds to step 133 ofdisplaying the current temperature estimate. On the other hand, it isdetermined in step 137 that the difference between the maximum andminimum temperature estimates during the preceding 2.0 seconds exceeds0.25° F., then the program returns to step 115 and the predictionprocess continues. Only when it is finally determined at step 137 thatthe maximum and minimum temperature estimates differ by less than 0.25degrees will the prediction process finally be concluded.

Returning to step 121, where it is determined whether or not the currenttemperature measurement remains above 94.5° F. or alternatively remainsabove 91.5° F. with no warming pulses being applied to the resistor 31,if it is determined ever that that condition is no longer being met,then it is deduced that the probe 15 has been removed from the patient'smouth and that the warming procedure described above must be resumed.Thus, in step 139, the program calculates an appropriate pulse durationfor the pulse-width modulation signal, using the formula set forthabove. Then, in step 141, the pulse is applied to the resistor 31. Theprogram then returns to the step 115 of waiting for the predict clock todecrement to zero.

In most cases, the thermometer 11 determines that a temperature estimateoutside the 97° F. to 99.5° F. range is indeed valid when the end of the3.6 second time period is first reached. More than 3.6 secondsordinarily is required only when the probe 15 is not properly seatedwithin the patient's mouth or rectum, or otherwise is being moved aboutexcessively.

It should be appreciated from the foregoing description that the presentinvention provides an improved prediction-type medical thermometer thatprovides an accurate estimate of a patient's temperature insubstantially reduced time as compared to prior thermometers of thiskind. This improved performance is achieved by using a special hollowprobe tip having a low heat capacity and incorporating a resistiveheater that is actuated only upon removal of the probe from a basehousing. The heater regulates the temperature to a value of about 93° F.After the probe is applied to the patient, a microprocessor periodicallysamples the thermistor and estimates the patients temperature based onthe successive samples. If a predetermined number of successivetemperature estimates lie within a predetermined error range, thesampling is terminated and the most recent estimate is displayed as thepatients temperature. In addition, when that estimate lies outside of anormal temperature range, an increased number of successive estimateslying within a predetermined error range are required before thethermometer displays the estimated temperature.

Although the invention has been described in detail with reference onlyto the presently preferred embodiment, those skilled in the art willappreciate that various modifications can be made without departing fromthe invention. Accordingly, the invention is defined only by thefollowing claims.

We claim:
 1. A medical thermometer comprising:an elongated probe havinga longitudinal axis and includingan elongated base having a remote end,and a hollow, thin-walled metallic tip having a closed end and an openend sized for secure attachment to the remote end of the elongated base,such that an elongated cavity is defied beyond the remote end of theelongated base and within the metallic tip; and a thermistor bonded toan inside wall of the probe's hollow metallic tip, within the elongatedcavity, the thermistor generating an electrical signal that variesaccording to the thermistor's temperature; wherein the elongated cavitydefined within the probe's hollow metallic tip is dimensioned such thatthe cavity's length, measured longitudinally from the remote end of theprobe's elongated base to the closed end of the hollow metallic tip, issubstantially longer than the cavity's transverse width measured at thesite where the hollow metallic tip attaches to the remote end of theelongated base, to inhibit the conduction of heat along the hollowmetallic tip to the elongated base, and wherein the elongated cavity isfree of any structure that provides a substantial heat conduction pathbetween the thermistor and the elongated base, whereby the temperatureof the thermistor closely follows the temperature of any surface thatcontacts the metallic tip.
 2. A medical thermometer as defined in claim1, wherein the hollow metallic tip is formed of stainless steel and hasa substantially uniform thickness less than or equal to about 0.1millimeters.
 3. A medical thermometer as defined in claim 1, and furthercomprising:a resistor bonded to the inside wall of the probe's hollowmetallic tip, within the elongated cavity, at a location spacedcircumferentially from the thermistor; and an electrical circuit forselectively applying an electrical current to the resistor, to warm theprobe's hollow metallic tip to a selected temperature.
 4. A medicalthermometer as defined in claim 3, wherein the thermistor and theresistor are bonded to the inside wall of the probe's hollow metallictip at substantially diametrically opposed positions.
 5. A medicalthermometer as defined in claim 1, wherein the hollow metallic tipincludes a cylindrical section that secures to the remote end of thebase and a frusto-conical section at its remote end, the thermistorbeing bonded to the frusto-conical section.
 6. A medical thermometer asdefined in claim 1, wherein the thermistor is bonded to an inside wallof the probe's hollow metallic tip at a location spaced from the remoteend of the elongated base.
 7. A medical thermometer comprising:anelongated probe having a longitudinal axis and including an elongatedbase having a remote end, and a hollow, thin-walled metallic tip havinga cylindrical section at one end that secures to the remote end of thebase and a frusto-conical section at an opposite end, such that anelongated cavity is defined beyond the remote end of the elongated baseand within the metallic tip; a thermistor located within the cavity ofthe metallic tip and bonded to the frusto-conical section, thethermistor generating an electrical signal that varies according to thethermistor's temperature; a resistor bonded to the inside wall of theprobe's hollow metallic tip, within the elongated cavity, at a locationspaced substantially diametrically opposed from the position of thethermistor; and an electrical circuit for selectively applying anelectrical current to the resistor, to warm the probe's hollow metallictip to a selected temperature; wherein the elongated cavity definedwithin the probe's hollow metallic tip is dimensioned such that thecavity's length measured from the remote end of the probe's elongatedbase to the closed end of the hollow metallic tip, is substantiallylonger than the cavity's transverse width, measured at the site wherethe hollow metallic tip attaches to the remote end of the elongatedbase, to inhibit the conduction of heat along the hollow metallic tip tothe elongated base, and wherein the elongated cavity is free of anystructure that provides a substantial heat conduction path between thethermistor and the elongated base, whereby the temperature of thethermistor closely follows the temperature of any surface that contactsthe metallic tip.
 8. A medical thermometer as defined in claim 7,wherein the hollow metallic tip is formed of stainless steel and has asubstantially uniform thickness less than or equal to about 0.1millimeters.